This invention relates to a case hardening steel having good grain coarsening properties during carburization, to a method for producing the steel, and to a blank material for carburized parts.
Gear-wheels, bearing parts, rolling parts, shafts. and constant velocity joint parts are normally manufactured by a process using medium-carbon steel alloy for mechanical structures prescribed by, for example, JIS G 4052, JIS G 4104, JIS G 4105 and JIS G 4106 that is cold forged (including form rolling), machined to a specified shape and carburization hardened. Because cold forging produces a good product surface layer and dimensional precision, and results in a better yield, with a lower manufacturing cost, than hot forging, there is an increasing trend for parts that were conventionally produced by hot forging to be produced by cold forging which, in recent years, has produced a pronounced increase in the focus on carburized parts manufactured by the cold forgingxe2x80x94carburizing process. A major problem with carburized parts is reducing heat treatment strain. This is because a shaft that warps as a result of strain from heat treatment can no longer function as a shaft, or in the case of gear-wheels or constant-velocity joint parts, high strain from heat treatment can cause noise and vibration. The major factor in such heat-treatment induced strain is grain coarsening produced during the carburizing. In the prior art, grain coarsening has been suppressed by annealing after cold forging and before carburization hardening. With respect to this, in recent years there is a strong trend toward omitting the annealing as a way of reducing costs. Therefore, there has been a strong need for steel in which grain coarsening does not occur even if the annealing is omitted.
Bearing and rolling parts that have to take a high contact stress are subjected to deep carburization. As deep carburization requires an extended period of time ranging from ten-plus hours to several tens of hours, it gives rise to another important issue, that of reducing the carburization time for the purpose of saving energy. One effective way of reducing the carburization time is to use a higher carburizing temperature. Carburization is normally performed at around 930xc2x0 C. The problem with performing carburization at a higher temperature, in the range of 990 to 1090xc2x0 C., is that it results in grain coarsening and a lack of the necessary material qualities, such as rolling fatigue characteristics and the like. Thus, there is a demand for case hardening steel that is suitable for high-temperature carburizing, that is, the grains of which are not coarsened by high-temperature carburizing. Many of the bearing and rolling parts that have to take a high contact stress are large parts that are normally manufactured by the steps of hot forging bar steel, heat treatment such as normalizing or the like, if required, machining, carburization hardening, and, if required, polishing. To suppress grain coarsening during carburizing, following the hot forging step, that is, when the parts are still blanks, it is necessary to optimize a material for suppressing the grain coarsening.
For this, JP-A-56-75551 discloses steel for carburizing comprising steel containing specific amounts of Al and N that is heated to not less than 1200xc2x0 C. and then hot worked, whereby even after it has been carburized at 980xc2x0 C. for six hours it is able to maintain fine grains, with the core austenite grains being fine grains having a grain size number of not less than six. However, the grain coarsening suppression ability of the steel is not stable and, depending on the process used to produce the steel, the steel may be unable to prevent grain coarsening during carburizing.
JP-A-61-261427 discloses a method of manufacturing steel for carburizing in which steel is used that contains specific amounts of Al and N, wherein after the steel has been heated to a temperature corresponding to the amounts of Al and N, then hot rolled at a finishing. temperature of not more than 950xc2x0 C., the precipitation amount of AlN is not more than 40 ppm and the ferrite grain size number is from 11 to 9. Again, however, the grain coarsening suppression ability of the steel is not stable and, depending on the process used to produce the steel, the steel may be unable to prevent grain coarsening during carburizing.
JP-A-58-45354 discloses a case hardening steel containing specified amounts of Al, Nb and N. Again, however, the ability of the steel to suppress grain coarsening is not stable, so that in some cases grain coarsening is suppressed, and in other cases it is not. Moreover, in the examples the steel is described as having a nitrogen content of not less than 0.021%. If anything, that would have the effect of worsening the grain coarsening properties, making the steel susceptible to cracking and blemishes during the production process, in addition to which, because of the hardness, the material would have poor cold workability.
Thus, the above methods are not able to stably. suppress grain coarsening during carburization hardening, and therefore are not able to prevent strain and warping. With respect also to bearing and rolling parts that are subjected to high contact stresses, there are no examples in which such parts that have been subjected to deep carburizing by carburizing at a high temperature exhibit adequate strength properties. That is, there are no prior examples of blank materials for carburized parts or case hardening steel suitable for high-temperature carburization.
An object of the present invention is to provide case hardening steel with low heat-treatment strain having good grain coarsening prevention properties during carburization, a method of producing the steel, and, with respect to the production of carburized parts produced in the hot forging process, blank material for carburized parts that are able to prevent grain coarsening even during high-temperature carburizing and have adequate strength properties.
To attain the above object, the present inventors investigated what the dominant factors in grain coarsening were, and clarified the following points.
1. Even though steels may have the same chemical composition, in some cases they may be able to suppress grain coarsening and in other cases they may not be able to: grain coarsening cannot be prevented just by limiting the chemical composition. An important factor, apart from the chemical composition, is the state of precipitation of carbonitrides after the steel has been hot rolled or hot forged.
2. A key to preventing grain coarsening during carburization is, during carburization heating, to effect dispersion of a large amount of fine AlN and Nb(CN) as pinning particles.
3. To ensure a stable manifestation of the pinning effect of the Nb(CN) during carburization heating, the hot rolled or hot forged steel needs a prior fine precipitation of at least a given amount of Nb(CN). Moreover, if coarse AlN is precipitated or TiN or Al2O3 is present in the steel after the steel has been hot rolled or hot forged, it will form coarse Nb(CN) precipitation nuclei, impeding the fine precipitation of the Nb(CN). This being the case, it is necessary to keep the Ti content and O content as low as possible.
4. To ensure a stable manifestation of the pinning effect of the AlN during carburization heating, in contrast to Nb(CN), it is necessary to minimize the AlN precipitation amount in the steel in the hot rolled or hot forged condition. This is an essential requirement for achieving fine precipitation of the Nb(CN). Moreover, any TiN or Al2O3 that is present in the steel after the steel has been hot rolled or hot forged will form AlN precipitation nuclei, increasing the amount of AlN precipitation, so in this case, too, the Ti and O contents have to be minimized.
5. Even if carbonitrides are controlled as described, any admixture of bainitic structure in the steel after hot rolling will promote grain coarsening during carburization heating.
6. Moreover, grain coarsening will occur more readily during carburization heating if the ferrite grains in the steel following hot rolling are excessively fine.
7. In order to minimize the AlN precipitation amount in the steel in the hot rolled condition, the steel has to be heated to a high temperature for the hot rolling.
8. Prior fine precipitation of at least a given amount of Nb(CN) in the steel that has been hot rolled can be ensured by optimizing the hot rolling temperature and the cooling conditions used after the hot rolling. That is, the Nb(CN) is occluded in the matrix by heating the steel to a high temperature for the hot rolling, and after the steel has been hot rolled, the Nb(CN) can be finely dispersed in large amounts by cooling slowly in the Nb(CN) precipitation temperature region.
The present invention was achieved based on the above novel findings. The gist of the present invention is as follows.
The invention of claims 1 to 4 is, a case hardening steel having good grain coarsening prevention properties during carburization characterized in that said steel comprises, in mass%,
0.1 to 0.4% C,
0.02 to 1.3% Si,
0.3 to 1.8% Mn,
0.001 to 0.15% S,
0.015 to 0.04% Al,
0.005 to 0.04% Nb,
0.006 to 0.020% N,
one, two or more selected from
0.4 to 1.8% Cr,
0.02 to 1.0% Mo,
0.1 to 3.5% Ni,
0.03 to 0.5% V,
and in which
P is limited to not more than 0.025%,
Ti is limited to not more than 0.010%, and
O is limited to not more than 0.0025%,
with the balance being iron and unavoidable impurities,
the steel, following hot rolling, having a Nb(CN) precipitation amount of not less than 0.005% and an AlN precipitation amount that is limited to not more than 0.005%,
and that also,
following hot rolling, the matrix of the steel contains not less than 20 particles/100 xcexcm2 of Nb(CN) of a particle diameter of not more than 0.1 xcexcm,
and that also,
following hot rolling, the bainite structure fraction of the steel is limited to not more than 30%,
and that also,
following hot rolling, the steel has a ferrite grain size number of from 8 to 11.
The invention of claims 5 to 7 is, a method of producing the above steel characterized in that the steel is heated to a temperature of not less than 1150xc2x0 C., maintained at that temperature for not less than 10 minutes, and hot rolled to form wire or bar steel, and that also,
after the steel is hot rolled the steel is slowly cooled between 800 and 500xc2x0 C. at a cooling rate of not more than 1xc2x0 C./s,
and that also,
the steel is hot rolled at a finishing temperature of 920 to 1000xc2x0 C.
The invention of claims 8 and 9 is, a steel blank material for carburized parts having good grain coarsening prevention properties during carburization characterized in that said blank material comprises, by mass,
0.1 to 0.40% C,
0.02 to 1.3% Si,
0.3 to 1.8% Mn,
0.001 to 0.15% S,
0.015 to 0.04% Al,
0.005 to 0.04% Nb,
0.006 to 0.020% N,
one, two or more selected from
0.4 to 1.8% Cr,
0.02 to 1.0% Mo,
0.1 to 3.5% Ni,
0.03 to 0.5% v,
and in which
P is limited to not more than 0.025%,
Ti is limited to not more than 0.010%, and
O is limited to not more than 0.0025%,
with the balance being iron and unavoidable impurities, the steel blank material, following hot forging, having a Nb(CN) precipitation amount of not less than 0.005% and an AlN precipitation amount that is limited to not more than 0.005%,
and also that,
following hot forging, the matrix of the steel contains not less than 20 particles/100 xcexcm2 of Nb(CN) of a particle diameter of not more than 0.1 xcexcm.